Evaluation of the potential probiotic <scp><i>Bacillus subtilis</i></scp> isolated from darkbarbel catfish (<scp><i>Pelteobagrus fulvidraco</i></scp>) on growth performance, serum immunity, and disease resistance of <scp><i>Aeromonas hydrophila</i></

This study aimed to identify potential probiotic strains of Bacillus subtilis from healthy fish gut microbiota for application in aquaculture. The effects of dietary B. subtilis administration on growth performance, serum enzyme activity, immune gene expression, and disease resistance in darkbarbel catfish (Pelteobagrus fulvidraco) were investigated. The isolate, identified through gene sequencing and biochemical tests, demonstrated resilience to pH 3.0% and 6.0% bile, and exhibited extracellular protease, cellulose, lipase, and amylase production. Darkbarbel catfish were fed diets with varying B. subtilis concentrations (0 CFU/kg [T0], 10⁷ CFU/kg [T1], 10⁸ CFU/kg [T2], and 10⁹ CFU/kg [T3]). After 8 weeks, significant increases (p < 0.05) were observed in final body weight, weight gain rate, specific growth rate, serum lysozyme, serum superoxide dismutase, alkaline phosphatase, and total antioxidant capacity, whereas malondialdehyde levels significantly decreased. Feeding darkbarbel catfish with B. subtilis diets increased immunoglobulin M (IgM) and C3 gene expression (p < 0.05), indicating a positive impact on the fish's immune system. The strain upregulated interleukin 10 (IL-10) and transforming growth factor-β (TGF-β) expression and downregulated TNF-α and IL-1β, suggesting potential anti-inflammatory effects. Following a 7-day challenge with Aeromonas hydrophila, fish fed with B. subtilis exhibited lower mortality, with higher survival rates in the T2 and T3 groups. In conclusion, supplementing darkbarbel catfish diets with B. subtilis effectively enhances growth performance, immune response, and disease resistance.